1970 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 52, NO. 8, AUGUST 2004 Ground Influence on the Input Impedance of Transient Dipole and Bow-Tie Antennas Andrian Andaya Lestari, Alexander G. Yarovoy, Member, IEEE, and Leo P. Ligthart, Fellow, IEEE Abstract—In this paper, the influence of a lossy ground on the input impedance of dipole and bow-tie antennas excited by a short pulse is investigated. It is shown that the ground influence on the input impedance of transient dipole and bow-tie antennas is signif- icant only for elevations smaller than of the wavelength that corresponds to the central frequency of the exciting pulse. Further- more, a principal difference between the input impedance due to traveling-wave and standing-wave current distributions is pointed out. Index Terms—Bow-tie antenna, dipole antenna, input impedance, transient antenna. I. INTRODUCTION D IPOLE and bow-tie antennas are employed in many transient applications such as impulse ground penetrating radar (GPR) for transmitting short transient pulses. The large antenna bandwidth required to transmit such pulses with min- imal distortion (e.g., antenna ringing) is usually obtained by the application of resistive loading [1], [2]. As resistive loading substantially reduces radiation efficiency [1], it is essential to achieve maximum power transfer from the generator to the antenna, for which the input impedance of the antenna should be known. The input impedances of time-harmonic and transient an- tennas are principally different since the former is due to standing-wave current distribution, while the latter is due to traveling-wave current distribution. Publications with regard to the input impedance of time-harmonic dipole and bow-tie an- tennas near the ground are abundantly available in the literature. On the contrary, not much of the input impedance of transient dipole and bow-tie antennas near the ground has been reported. In the free-space case, significant contributions were given by Wu [3] and Carrel [4] who presented analytical expressions of the input impedance of transient dipole and bow-tie antennas, respectively. In this paper we analyze the input impedance of transient dipole and bow-tie antennas near a lossy ground. A numerical method to predict the input impedance of arbitrary metallic transient antennas in free space using the time-domain integral equation (TDIE) method has been demonstrated by Booker, et al. [5]. In their work the TDIE is Manuscript received March 7, 2003; revised August 20, 2003. This work was supported by the Dutch Technology Foundation (STW) under the projects “Improved Ground Penetrating Radar Technology” (1999–2000) and “Ad- vanced Re-Locatable Multisensor System for Buried Landmine Detection” (2001–2002). The authors are with the International Research Centre for Telecommunica- tions-Transmission and Radar (IRCTR), Delft University of Technology, Delft, The Netherlands (e-mail: a.lestari@irctr.tudelft.nl). Digital Object Identifier 10.1109/TAP.2004.832371 numerically solved by the method of moments (MoM) using the marching-on-in time approach. By neglecting end reflec- tions in the time-domain solution, the input impedance is given by the high-frequency limit of the frequency-domain solution obtained by Fourier transforming the mentioned time-domain solution. Unfortunately, when it comes to layered-medium problems, well-suited Green’s functions in space-time domain are not yet well documented. One of few developments of such Green’s functions has recently been reported for analyzing the response of a transient dipole in stratified media [6]. However, layered-medium Green’s function formulations in space-time domain which are directly applicable to surface-patch MoM methodologies, are not yet widely reported. The numerical analysis carried out in this work is based on the frequency-domain integral equation (FDIE) method, as the methods of solution for problems with layered media in fre- quency domain are already well established. The FDIE incor- porates a layered-medium Green’s function and is numerically solved by a surface-patch MoM scheme for metallic nonwire structures, whereas wire structures are approximated by narrow strips. The input impedance of the transient antennas is ob- tained by the Fourier transformation and a time-window tech- nique for excluding end reflections. An experimental analysis is performed to verify the computed results. II. NUMERICAL METHOD The work reported in this paper is based on a frequency- domain mixed-potential integral equation (MPIE) formula- tion. To account for the presence of the ground the dyadic Green’s function formulation C for layered-medium problems by Michalski and Zheng [7] is incorporated into the MPIE, which is numerically solved by MoM according to the trian- gular surface-patch methodology introduced by Rao, et al. [8]. Computation time is minimized by employing the efficient numerical implementation introduced in [9]. In this work the antennas are excited by a monocycle with 0.8-ns duration shown in Fig. 1(a). In Fig. 1(b) its normalized spectrum is given. The central frequency of this pulse is about 1 GHz and the dB levels are found at frequencies 424 MHz and 1.670 GHz. It can be seen in Fig. 1(b) that the spectrum of the exciting pulse is essentially contained within the 0–5 GHz range. In this paper, we develop a nonstraightforward numerical method to predict the input impedance of a transient antenna in four steps as follows. 1) Antenna feed current is computed in the frequency domain with 1 Volt input voltage by means of the MoM 0018-926X/04$20.00 © 2004 IEEE